Under Pressure: FAR Part 91.144

A few weeks ago I was doing my normal routine preparing the airplane for a short haul from Memphis to Chattanooga when the altimeter setting floored me. “Altimeter 30.82,” the ATIS read. That was one mighty high pressure system we were under. In fact, 30.82 is a whole 900 foot difference from the standard altimeter setting of 29.92! This reminded me of the elusive FAR Part 91.144.

(a) Special flight restrictions. When any information indicates that barometric pressure on the route of flight currently exceeds or will exceed 31 inches of mercury, no person may operate an aircraft or initiate a flight contrary to the requirements established by the Administrator and published in a Notice to Airmen issued under this section.
(b) Waivers. The Administrator is authorized to waive any restriction issued under paragraph (a) of this section to permit emergency supply, transport, or medical services to be delivered to isolated communities, where the operation can be conducted with an acceptable level of safety.

In English, if the altimeter is near 31.00, the normal rules for setting the altimeter might not apply. It is the responsibility of the pilot in command to check the NOTAMs and comply with the special rules implemented for this weather condition. In fact, you can not legally fly unless you have complied with the NOTAM.

It is also important to keep in mind that not everyone will have checked the NOTAMs, meaning that you should be particularly suspicious when you hear that weekend-warrior state that he is “maneuvering at 3,000 feet”. He may have set his altimeter to the limit (31.00), or he may have assumed that it would be wise to fly around at 29.92. That’s a potential difference of 1,080 feet from your indication of 3,000 feet.